NM - Aerobic Metabolism

Question 1

What is the main function of the Krebs Cycle?

Answer 1

The oxidation of acetyl CoA to carbon dioxide (CO2) and water (H2O).

Question 2

Where does the Krebs Cycle occur?

Answer 2

In the mitochondrial matrix.

Question 3

In which tissues does the Krebs Cycle occur?

Answer 3

In all tissues with mitochondria (not red blood cells or white muscle fibers).

Question 4

Describe the reaction that links glycolysis to the Krebs Cycle (and could also be said to prepare the molecule for the Krebs Cycle).

Answer 4

CH3 CO COO- + CoA (+ NAD+) -> CH3 CO CoA + CO2 (+ NADH + H+)

It is the conversation of pyruvate to Acetyl CoA.

It is catalysed by pyruvate dehydrogenase.

Other cofactors:

• thiamine pyrophosphate (thiamine: vitamin B1)
• lipoic acid
• FAD (riboflavin: vitamin B2)
• Coenzyme A (panthothenic acid: vitamin B5)

Question 5

Outline the different products of the TCA/Krebs Cycle, what they become, and how many carbons they have, starting from the addition of Acetyl CoA.

Answer 5

Acetyl Coa (2C) + Oxaloacetate (4C) (catalysed by citrate synthase)
↓
Citrate (6C) 
↓
Isocitrate (6C)
↓ (- CO2)
α-Ketoglutarate (5C)
↓ (- CO2)
Succinyle-CoA (4C)
↓
Succinate (4C)
↓
Fumerate (4C)
↓ 
Malate (4C)
↓
Oxaloacetate (4C)

and the cycle restarts

Question 6

What does Coenzyme A do?

Answer 6

It is the enzyme responsible for moving the carbon atoms within the acetyl group to the citric acid cycle.

It forms thioester bonds with carboxylic acids. It is linked to a sulfur-containing compound, which is the reactive group, the SH group.

CoASH forms an ester with acetate, creating the SCoA.

Question 7

Describe the role of FAD as a H acceptor.

Answer 7

FAD = Flavin Adenine Dinucleotide

The active group is riboflavin, which is vitamin B2.

It is also involved in the reaction, and it takes up two hydrogen ions.

Question 8

List the different steps in the Krebs Cycle and describe them where necessary.

Answer 8

(1) Condensation reaction:
Oxaloacetate (4C) and Acetyl CoA (2C) combine to form Citrate (6C), catalysed by citrate synthase.

(2) Isomerisation:
Citrate (6C) is converted to Isocitrate (6C), catalysed by Aconitase.

This reaction requires iron, and it reversible. Aconitate is an intermediate for the reaction, hence the enzyme name.

(3) First loss of CO2:
Isocitrate (6C) is converted to α-Ketoglutarate (5C), catalysed by Isocitrate dehydrogenase.

NADH is formed, so this reaction generates energy.

(4) Second loss of CO2:
α-Ketoglutarate (5C) is converted to Succinyl CoA (4C), catalysed by Ketoglutarate dehydrogenase.

NADH is also formed.

(5)   Trapping thioester bond  energy as GTP:
Succinyl CoA (4C) is converted to Succinate (4C), catalysed by Succinate thiokinase. 

GTP is created in this reaction.

(6) Conversion of succinate to fumarate:
Succinate (4C) is converted to Fumarate (4C), catalysed by Succinate dehydrogenase.

The compound is oxidised, and we get a reduced compound of FADH2.

(7) Conversion of fumarate to malate:
Fumarate (4C) is converted to Malate (4C), catalysed by Fumerase.

(8) Conversion of malate to oxaloacetate:
Malate (4C) is converted to Oxaloacetate (4C), catalysed by Malate dehydrogenase.

NADH is made from the reaction.

Question 9

What is the electron transport chain?

Answer 9

The electron transport chain is a series of protein complexes located at the inner membrane of the mitochondria.

Question 10

As an example, describe the oxidation of NADH to NAD+.

Answer 10

The oxidation of NADH to NAD+ occurs by transfer of 2H to the carriers of the cytochrome chain.

The electrons need to be channelled from a high energy state to a low energy state. So the NADH is oxidised, and the electrons migrate down the ETC (electron transport chain), and lose energy that is used to pump protons.

Then the electrons are put on oxygen – a reduction reaction – to form water.

Question 11

Describe the overall process at the mitochondrial electron transport chain.

Answer 11

The inner side of the mitochondria is where the Krebs cycle and FA oxidation take place.

NADH is oxidised to NAD+ in Complex I, and the electrons are moving across the membrane. FADH2 is oxidised to FAD+ in Complex II, and a similar thing happens with the electrons.

Complex IV is important for putting the electrons on oxygen to form water.

Two facts are important. Namely, the fact that these complexes are proton pumps. Thus, we are pumping protons from the matrix into the mitochondrial space to maintain a proton gradient.

This gradient is used in the final complex, the ATP synthase. It takes the H+ ions from the mitochondrial space, and uses its diffusion down its gradient to start up a molecular motor, so to speak. The energy from that movement is used to put phosphate on ADP to make it ATP.

Question 12

What are the energy yields of the TCA cycle?

Answer 12

3 enzyme reactions produce NADH and H+.
1 enzyme reaction produces FADH 2.
1 enzyme reaction produces GTP.

Thus:
3 x 2.5
1 x 1.5
1 x GPT
\_\_\_\_\_\_
10 ATP

This is 5 times more than what you get in glycolysis.

Question 13

Which 3 enzymes of the TCA cycle catalyse irreversibly?

Answer 13

3 enzyme steps are highly exergonic & irreversible:

• citrate synthetase (Oxaloacetate + Acetyl CoA -> Citrate)
• isocitrate dehydrogenase (Isocitrate -> α-Ketoglutarate)
• ketoglutarate dehydrogenase (α-Ketoglutarate -> Succinyl CoA)

Question 14

Which substances activate/inhibit the three key enzymes of the TCA cycle?

Answer 14

citrate synthetase:

• NADH inhibits
• succinyl CoA inhibits

isocitrate dehydrogenase:

• ADP - activates
• NADH - inhibits

ketoglutarate dehydrogenase:

• NADH - inhibits
• succinyl CoA inhibits